High frequency and wide microwave bandwidth field effect transistors (FETs) are useful for many applications, such as, high performance commercial communications and military systems and automotive collision avoidance systems. The maximum operating frequency of a FET is dependent upon the length of the gate electrode positioned between the source and drain regions of the FET. For example, gate capacitance can be lowered by shortening the gate length which increases the maximum operating frequency. However this reduces the cross sectional area of the gate which causes gate resistance to rise. High gate resistance leads to degradation of the microwave noise figure and degradation of gain at microwave frequencies. Thus it is advantageous to fabricate a gate with a short gate length without significant reduction in the overall cross-sectional area of the gate.
A gamma gate (often referred to as a "mushroom gate") is a gate electrode often used in FET devices which has a cross section shaped like the Greek letter gamma (.GAMMA.). The gamma gate has been found to be desirable because the gate length at the interface with the wafer surface is very short while a desirable cross-sectional area is maintained with the larger cap.
Reliable and affordable fabrication of such gate electrodes has proven difficult particularly when reducing gate lengths to sub-quarter micron dimensions. Many prior art methods employ expensive or low yield techniques such as electron beam (e-beam) lithography, photoresist reflow or optical steppers using phase-shift mask techniques. These methods suffer from poor cost effectiveness (electron beam), low throughput (electron beam), and poor uniformity and repeatability (photoresist reflow) and process immaturity (phase-shift mask).